I requested this a while back. Since no one has released a hack like this, I worked out how to do it.

NOTE: This hack automatically shows the age of the person calculated from the birthday entered (it’s not a separate input). If there is no birthdate or birthyear, the age doesn’t show up automatically.

1) Postbit hack

This hack basically places the member’s age underneath his join date (or wherever you want) in the postbit, like this:

Registered: July 2000
Age: 18
Location: USA
Posts: 102

If the member has not entered his birth-year, then the age will NOT show up at all, like this: